Forty of the world’s leading climate scientists have signed an open letter demanding global leaders take bolder action against climate change (see full list below).

The joint statement – initiated by WWF and endorsed by recognised climate luminaries such as Sir John Houghton, former chair of the Intergovernmental Panel on Climate Change (IPCC) – calls for industrialised countries to make a commitment, at the UN Climate summit in Copenhagen, to cut carbon emissions by at least 40% below 1990 levels by 2020.

Saleemul Huq, one of the signatories, IPCC author and Senior Fellow in the Climate Change Group at the International Institute for Environment and Development, said: “The scientific evidence now indicates that even a rise in temperature of 2°C will entail considerable hardships for poor and vulnerable people around the world, especially those living on low-lying islands and coasts.

“So a 40% reduction in emissions is the very least required to provide a better chance of avoiding devastation for these countries and communities.”

WWF’s Head of Climate Change, Keith Allott, says: “As the UK government rallies the EU to step up to the mark ahead of Copenhagen, it’s time for Gordon Brown and other world leaders to turn words into action.”

Dr Dave Reay, another of the signatories, IPCC contributor and Senior Lecturer in Carbon Management, Edinburgh University, said: "The scientific evidence of climate change from around the world is providing a clear and urgent call for action.

“If we are to be successful in preventing the worst impacts of climate change then world leaders from the industrialised nations must commit to reducing emissions by at least 40 per cent by 2020. The meeting in Copenhagen later this year is hugely important in putting the world on a path that leads us away from dangerous climate change."

The WWF say key meetings that will shape the global climate deal take place in coming weeks, including the UN General Assembly in New York and the G20 Heads of State meeting inPittsburgh.

Campaigners say it is vital that the politicians attending take note of such timely advice from the world’s scientific community.

The scientists’ statement on 40% emissions reduction target for developed countries follows…

Copenhagen climate targets must be more ambitiousAt the UN Climate Conference in Copenhagen this December, world leaders have the opportunity to agree a historic global climate deal. To avoid dangerous climate change, the deal must be based on the most up-to-date scientific understanding of the emissions reductions required, with obligations divided equitably between developed and developing countries. This means that developed countries must reduce greenhouse gas emissions by at least 40% below 1990 levels by 2020.

Copenhagen represents our best chance to avert the worst impacts of climate change on people, species and ecosystems. More than 120 countries, including the members of the G8, the EU, and key emerging economies such as China, South Africa and Mexico, agree that the rise in global temperature must stay well below 2°C. Beyond this point climate impacts will be more severe, with the risk of crossing ‘tipping points’ with dangerous and irreversible effects.

To stand a good chance of achieving this goal, the last Intergovernmental Panel on Climate Change report (2007) recommended that developed countries should reduce emissions by 25-40% on 1990 levels by 2020. Yet more recent evidence shows that only reductions at the top end of this range will be sufficient to avoid the worst impacts of climate change.

Developed countries have so far committed to reduce greenhouse gas emissions by only 10-16% by 2020, a level dangerously inconsistent with their commitment to the 2°C target. The latest scientific evidence clearly shows that these countries must increase their ambition and reduce emissions by 40% by 2020 to maintain a credible ambition of avoiding dangerous climate change.

Prof Abhijit Mitra, IndiaDepartment of Marine Science, University of Calcutta

Dr Carlos Afonso Nobre, BrazilIPCC Lead Author, Head of the Scientific Committee of the International Geosphere-Biosphere Program, Co-ordinator of the Centre for Earth System Science at the National Institute for Space Research and Executive Secretary of the Brazilian Network for Climate Change Research

Pan Jiahua, ChinaIPCC advisor to Working Group III and Executive Director of Research Centre for SustainableDevelopment at the Chinese Academy of Social Sciences

Dr Suzana Kahn Ribeiro, BrazilVice-Chair of IPCC Working Group III and Coordinating Lead Author of IPCC 4th Assessment Report

Dr Luis Pinguelli Rosa, BrazilHead of Brazilian Forum on Climate Change and Director at Alberto Luiz Coimbra Institute for Post-Graduation and Research in Engineering, Federal University of Rio de Janeiro

Dr Jim Salinger, New ZealandLead Author for IPCC 3rd and 4th Assessment Reports, Honorary Associate Professor, School of Environment, University of Auckland and President of the World Meteorological Society’s Commission for Agricultural Meteorology

Dr Roberto Schaeffer, BrazilIPCC Lead Author and Researcher at Federal University of Rio de Janeiro